Vehicles for cargo transport include a tractor-trailer that includes a tractor having a drive source and a trailer towed by the tractor. The trailer is coupled to the tractor with a coupler.
A large-size trailer is provided with axles, and wheels are arranged at the opposite ends of each axle. Thus, when the trailer travels in a state in which the load amount is large, e.g., when a cargo with the maximum capacity amount is loaded on the trailer, the contact pressure of each wheel becomes less than or equal to a predetermined value. However, when the trailer travels in a state in which no cargo is loaded or in which the load amount is small, traveling with all the wheels on the ground increases fuel consumption and wears the road surface.
Thus, a lift axle has been developed that lifts wheels attached thereto from the road surface. Specifically, the lift axle is one of the axles provided for a trailer and is capable of ascending and descending according to the load amount of the cargo (e.g., refer to Patent Document 1).